1. Field of the Invention
The present invention relates to a device which simulates the sound of breaking glass and, in particular, to a device which is actuated by pressure.
2. Description of the Related Art
Intrusion alarms are devices which generate an alarm signal when an unauthorized entry into a protected structure is detected. One common method for gaining access into the protected structure is to break out a window. In response to this, glass break detectors have been developed to generate an alarm signal when the sound of breaking glass is detected.
When glass mounted in the wall of a room is broken by impact, many variables affect the sound that is produced. Some of these variables are the type of glass, its size, the mounting method, and the acoustic properties of the room. However, for all glass broken by impact, the acoustic signal that results is a short-term event typically lasting from one-half to three seconds. The peak amplitudes are concentrated in the initial portions of the signal. The frequency spectrum of the acoustic signal is very wide, ranging from as low as 3 Hz to well over 20 kHz. The low-frequency components of the signal are caused by the initial displacement of the glass as it rebounds from the blow. If the mounting frame and wall are flexible, they may contribute to the low-frequency components as well. The high-frequency components are caused by the emissions associated with the actual fracturing of the glass and secondarily by collisions of glass fragments with each other and with barriers in the room.
All glass break detectors which rely on detecting the sound of breaking glass operate by selectively detecting one or more of the frequency components associated with the sound of breaking glass. Some glass break detectors listen only for a narrow band of frequencies in the high end of the spectrum while others listen for both the high and low frequencies. The low-frequency is caused by the flexing of the window immediately prior to its breakage. Glass break detectors, like model FG-730 manufactured by C & K Systems, Inc., require the two components to have a defined duration and arrive nearly simultaneously before the acoustic signal is identified as a glass break. Using several glassbreak frequencies improves false-alarm immunity.
When a glass break detector is installed, it should be tested because the acoustic properties of a room may affect the range of the glass break detector. In a room containing highly absorptive materials such as carpets, drapes, and acoustical tile ceilings, the detection range for high-frequency components will be much less than in a room with hard, reflective walls, floors, and ceilings. Although ordinary absorptive materials do not affect low-frequency components of the acoustic signal (below about 500 Hz), the geometry of the room does. Rooms with larger enclosed volumes cause greater attenuation of low-frequency components with distance than occurs in smaller rooms.
In addition to the acoustic properties of the room, tolerance of components in the glass break detector may cause the actual detection range to be less than what is expected.
For glass break detectors which detect only the high-frequency sound components, relatively simple simulators can be fashioned which simulate the high-frequency sound components. For glass break detectors which detect both the high and low frequency sound components, installation testing becomes much more difficult for two reasons. First, it is difficult to design a compact device which can produce low-frequency sound components. Second, it is difficult to independently produce high and low frequency sound components which will arrive nearly simultaneously at a glass break detector.